In recent years, as lithium ion rechargeable batteries achieve higher output density and higher capacity, battery separator films having excellent oxidation resistance and withstand voltage characteristics have been demanded.
JP-A5-222237 describes a polyolefin microporous membrane produced by forming a gel-like composition by cooling a polyolefin solution obtained by melt-mixing a nucleating agent, a mixed solvent of a good solvent for polyolefin and a solvent having excellent dispersibility for the nucleating agent, and polyolefin; heating and stretching the gel-like composition; and then removing the remained solvent. Since a mixed solvent is used in such a polyolefin microporous membrane, the solvent is not uniformly dispersed in the polyolefin solution, thereby increasing both the maximum pore size and the mean flow pore size of the obtained polyolefin microporous membrane measured by a porometer and also reducing air permeation resistance. Therefore, for use as a separator of a battery, withstand voltage characteristics need to be further enhanced.
JP-A-2010-215901 and JP-T-2009-527633 disclose polypropylene porous films obtained by production methods known as dry methods. Specifically, these are methods which employ low temperature extrusion and high draft ratio during melt extrusion to control a lamella structure in a film that has been formed into a sheet and that is prior to stretching, and which form voids by stretching such a film. However, by such production methods, the pore structure tends to be uneven and a structure with locally arranged voids may be formed. In particular, there has been a problem of poor dielectric breakdown resistance characteristics due to increased pore size of the pores on the surface of the microporous membrane and uneven distribution thereof.
JP-A-6-223802 describes a microporous membrane formed from a mixture of a high molecular weight polyethylene and high molecular weight polypropylene. However, since such a microporous membrane is structured mainly by polyethylene, although the impedance thereof is low, further enhancement in oxidation resistance is required.